To meet a high-rate transmission requirement for next-generation mobile communications, LTE-A (Long Term Evolution-Advanced, Long Term Evolution-Advanced) has introduced a wireless relay technology. As a newly-introduced network node in LTE-A, an RN (Relay Node, relay node) functions as both a UE (User Equipment, user equipment) and an eNB (evolved NodeB, evolved base station). On one hand, the same as a conventional UE, the RN connects, through a DeNB (Donor eNB, Donor eNB) that provides an access service for the RN, to an MME (Mobility Management Entity, mobility management entity). On the other hand, after the RN changes to an eNB, UEs within the coverage of the RN connect to the MME through the DeNB. Therefore, when inter-MME handover occurs on the RN, that is, when S1 handover occurs, the UEs change their MMEs along with the RN, or may be attached to respective MMEs with the RN, in which case some UEs need to change their MMEs and some UEs do not need to change their MMEs. In a scenario where the S1 handover occurs on the RN, context transfer is needed to maintain continuity of a device connection.
When the S1 handover occurs on the RN, a source MME to which the RN is attached transfers a context of the RN to a target MME to which the RN will be attached, thereby ensuring continuity of an RN connection before and after the S1 handover occurs. The context of the RN at least includes information of the RN, such as security information, KASME, and KSI.
When the S1 handover occurs on the RN, the UEs within the coverage of the RN cannot be aware of the handover process. When a source MME to which a UE is attached changes, the source MME does not process a context of the UE. As a result, security information of the UE is lost in the process of the S1 handover that occurs on the RN, and therefore a target MME to which the UE will be attached cannot acquire the security information of the UE. Then security protection cannot be implemented for an NAS (Non-Access Stratum, non-access stratum) message between the UE and the target MME of the UE.